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Yet another keeping warm question

I know these questions get old, but I haven't seen this specific issue mentioned in the searches.

The other day I made an underquilt that is two layers. The top layer is a vellux blanket cut to size, and the bottom layer is a wal-mart mystery nylon. Maybe DWR. The two long sides, and one short side are sewn together leaving one short side open forming a pocket. This is to make the underquilt milti-weather capable. Just use as is for windy mild weather. Add a ccf pad, leaves, or extra clothes as necessary in colder weather. The open end is made so it doesn't come open once tied under the hammock. My question is, if I were to put a mylar blanket in the pocket, would it cause problems? I'm thinking that since the underquilt isn't a perfect fit, there will be openings for moisture to evaporate, but it will still keep wind from taking away my heat. Any input would really be appreciated since I don't get many opportunities to test out my gear overnight.

I'm not sure what vellux is, but I think as long as you have a reasonably thick layer of breathable material between you and the mylar, you should be OK unless it's humid out. And as you mentioned, air spaces will help with evaporation. Of course, they allow your body heat to escape along with the moisture too

"Physics is the only true science. All else is stamp collecting." - J. J. Thompson

I didn't know what vellux is before I saw it on the shelf, and still don't know much about it. It is 100% nylon, and warmer and lighter than fleece. It is fairly thin, but looks like a thin layer of nylon padding, made like ccf pad, with little nylon fibers sticking out like little hairs on both sides. It is a little stiff, so when it curves around the contours of the hammock it has little areas that bow out just a little bit because it doesn't follow the shape perfectly. I don't think the air gaps are big enough to cause excessive air circulation. I plan on sleeping in thermals, and covering up with a fleece blanket, so my main concern is just underside insulation. I would guess that there is maybe 1/4 inch of clothes, hammock, and vellux between the mylar blanket and my skin. The air pockets don't go all the way around the hammock. Just from about where the hammock curves around my ribs to the top. There are about 4 on each side, and maybe 1-2 inches wide at the opening, like little chimneys. I would think that since they don't go all the way around air wouldn't draft through.

I didn't know what vellux is before I saw it on the shelf, and still don't know much about it. It is 100% nylon, and warmer and lighter than fleece. It is fairly thin, but looks like a thin layer of nylon padding, made like ccf pad, with little nylon fibers sticking out like little hairs on both sides. It is a little stiff, so when it curves around the contours of the hammock it has little areas that bow out just a little bit because it doesn't follow the shape perfectly. I don't think the air gaps are big enough to cause excessive air circulation. I plan on sleeping in thermals, and covering up with a fleece blanket, so my main concern is just underside insulation. I would guess that there is maybe 1/4 inch of clothes, hammock, and vellux between the mylar blanket and my skin. The air pockets don't go all the way around the hammock. Just from about where the hammock curves around my ribs to the top. There are about 4 on each side, and maybe 1-2 inches wide at the opening, like little chimneys. I would think that since they don't go all the way around air wouldn't draft through.

Sounds like you're probably fine as far as condensation, especially if the material breathes pretty well.

Don't underestimate how much heat you can lose through those air pockets. I don't know what kind of temps you're planning on using this setup in, but in sub-freezing weather you can lose a LOT. I'd be less worried if the gaps were just in the middle - not much place for the air to go - but around the sides like you're describing they could be a problem. As you heat the air it's going to rise, and without blockage, it'll come right out the top of those pockets. I'd suggest trying to adjust the insulation so it fits better around the sides, or even add drawcord or elastic (shockcord works well) around the perimeter to seal up those gaps. I think you'll have a lot nicer results that way.

"Physics is the only true science. All else is stamp collecting." - J. J. Thompson

I will test it out as is, and if necessary I will try to get rid of the gaps. I have some scraps that i will weigh. I tried it out the other day when the wind was blowing about 20 mph (on a farm in an open field on top of a hill) and the temp was about 40 deg. F. Felt fine. Will try it near freezing temps and let everyone know. This vellux stuff seems real promising. My aunt has a couple of them, and she swears by it as a good blanket to keep warm around the house.

Of, forgot to add the dimensions. It is 4.5 feet wide by 6.5 feet long. Just enough to go from head to toe, and wrap up around me in my hammock.

You will be fine with the mylar blanket. My underquilt is currently a layer of Felt, then Mylar blanket (emergency blanket), layer of quilting bating and finaly the outer shell is 1.1 oz Ripstop nylon. I have had this down to ~25 deg with just my 30 deg bag inside. I have had no problem with condensation yet. My only worning is to be cairful that the mylar is loose so that as the material streches the mylar will not rip. (had to open on up and replace the mylar).